This invention relates to a system for automatically shifting a multi-speed transmission between high and low ranges.
Heavy vehicles are typically equipped with a multi-speed transmission operated by a manual stick shift to achieve selected speed ratios between an engine input and the output of the transmission. In one common type of transmission, several speed ratios are provided in a main gear box. An auxiliary or range gear box is associated with the main gear box, and provides a high and low range. In one standard type of transmission the main gear box may have five speed ratios that may be selected by the operator. The auxiliary gear box has a high and a low position. The five gears in the main box provide speeds 1-5 when the range box is in low, and speeds 6-10 when the range box is in high. In this way, a relatively high number of speed ratio options are provided to the driver, without requiring as many individual gears be provided in the transmission. Aspects of the invention disclosed in this application will be explained relative to the above transmission; however, the present invention does extend to other transmission types.
Typically, the driver stick shift is provided with a switch that allows an operator to order a range shift as the operator is completing a shift (in the above example, fifth gear to sixth gear). The range shift would also be required when the operator is downshifting from sixth gear to fifth gear. While this basic system has worked well, it would be desirable to reduce the number of operations a driver must make when operating the vehicle.
As an example, when shifting a manual transmission through a change requiring a range shift, an operator of a heavy vehicle must request a range shift, actuate the clutch pedal, adjust the engine speed to match that necessary at the next gear, reactuate the clutch pedal, and move the transmission into the next gear to be engaged. This operation occurs frequently during operation of a heavy vehicle, and an operator may sometimes neglect to order the range shift when required. Moreover, the controls and necessary operations for operating a heavy vehicle have become much more complex in recent years. Thus, there is a desire to reduce the number of operations a vehicle operator must make during normal vehicle operation.
To this end, systems have been proposed that automatically change the range when a control decides that the range shift is necessary. In general, these systems have not been practical, and have not dealt with many real world variables that will be encountered in operation of the vehicle. As an example, most of these systems assume that a control will be able to store the actual currently engaged speed ratio. The logic for automatically actuating the proposed prior art range shifts is based upon the assumption that the currently engaged gear will be known and stored in a memory. The identification of the current gear is based upon remembering each gear shift. In fact, operators may manually change speeds, or may "skip shift" or shift through multiple speeds at one time. It thus becomes difficult for an engine control system to count and "remember" precisely which gear is presently engaged.
Moreover, the proposed systems rely upon complex control parameters to identify when a shift is necessary. As an example, some of the proposed systems may monitor throttle inlet, vehicle speed, and other variables that do not relate directly to when a range shift may be necessary. As such, the proposed system may not always provide accurate information, and may result in an undesired range shift, or no range shift when one should be actuated. As such, the proposed prior art systems have not achieved practically reliable automatic range shifting.